1. Field of the Invention
This invention relates to a control system for internal combustion engines, and more particularly to a control system for an internal combustion engine equipped with an evaporative emission control system for temporarily storing evaporative fuel generated from a fuel tank and purging the same into an exhaust passage of the engine to thereby prevent evaporative fuel from being emitted into the atmosphere.
2. Prior Art
Conventionally, an internal combustion engine (hereinafter simply referred to as "the engine") is widely used which has a control system for controlling an evaporative emission control system for storing evaporative fuel generated from fuel containers, such as a fuel tank and a float chamber of a carburetor, in a canister charged with an adsorbent, such as activated carbon, when the engine is in stoppage, and introducing the stored evaporative fuel via a purging passage into an intake passage of the engine by utilizing negative pressure created within the intake passage (purging) when the engine is in operation, thereby allowing evaporative fuel to be consumed by the engine as part of fuel supplied thereto.
According to a control system of this kind, when a throttle valve of the engine is fully closed so that negative pressure drastically increases within the intake passage, the air-fuel ratio of a mixture supplied to a combustion chamber of the engine becomes excessively enriched because purged evaporative fuel remaining in the purging passage and the intake passage is supplied to the combustion chamber while the amount of intake air supplied to the combustion chamber is very small. On such an occasion, due to shortage of oxygen in exhaust gases from the engine, noxious components of the exhaust gases, such as HC and CO, cannot be fully purified by a three-way catalyst arranged in the exhaust passage, resulting in degraded exhaust emission characteristics of the engine.
To solve this problem, a control system has been proposed by Japanese Laid-Open Patent Publication (Kokai) No. 1-273863, which interrupts purging of evaporative fuel immediately when the throttle valve is fully closed, but continues supply of fuel by injection over a predetermined time period to allow purged evaporative fuel remaining in the intake system to be consumed by combustion in the combustion chamber, and then interrupts supply of fuel to the engine (fuel cut).
However, according to the proposed control system, the fuel supply by injection is continued over the predetermined time period after the throttle valve is fully closed, which leads to wasteful consumption of fuel and also degraded responsiveness of the engine operation to operation of the throttle valve.